In part the reason that PHP8 (and it was 7 that had the quantum leap in perf) are now so fast is precisely because of Hack - it was easy to accept the status quo on performance until Hack showed there really was a lot of performance left on the table.

For me the biggest win was the changes they made to how arrays are stored in memory, I saw some systems drop by half in memory usage and had to change basically nothing - those kinds of wins are rare.